Research Review: Cadets Maintain Lean Body Mass Despite Prolonged Caloric Deficit During 44-Week Training Course

By Jackson Mann

BLUF (Bottom Line Up Front)

Both male and female officer cadets completing a 44-week British Army training course experienced sustained negative energy balance, with men showing significantly larger energy deficits due to higher total energy expenditure. Despite this, no significant lean mass loss occurred, and key metabolic and endocrine markers remained largely stable or adaptively regulated.

Purpose of the Study

This study aimed to assess sex-specific differences in energy balance, body composition, and metabolic and endocrine responses across a prolonged (44-week) British Army officer cadet training program. Researchers sought to understand whether men and women respond differently to sustained energy deficits commonly encountered during military training, and whether these deficits lead to negative outcomes such as lean mass loss or endocrine dysfunction.

Methods

The study followed 23 officer cadets (9 men and 14 women) enrolled in the British Army’s 44-week Commissioning Course. This program was divided into three 14-week training terms (T1–T3), each consisting of physical training, academic instruction, and field-based operational exercises. Energy intake was measured using 10-day weighed food logs during each term, while total energy expenditure was assessed using doubly labeled water (DLW) over the same 10-day periods. This method involves ingesting water labeled with stable isotopes of hydrogen and oxygen, then measuring the rate of isotope elimination through urine to estimate carbon dioxide production—an indirect but highly accurate measure of energy expenditure in individuals. Body composition was evaluated using dual-energy X-ray absorptiometry (DXA) scans taken at baseline and at the end of each term. Blood samples were collected at baseline, and again at the end of terms 2 and 3, to assess metabolic and endocrine markers such as leptin, testosterone, thyroid hormones (T3, T4, TSH), insulin-like growth factor I (IGF-I), and cortisol. Statistical analysis was conducted using linear mixed-effects models, with significance defined as P ≤ 0.05.

Key Findings

Men had significantly higher absolute energy intake and expenditure than women across all terms. However, because their energy intake did not scale proportionately to their expenditure, men experienced a larger negative energy balance than women. For example, during term 2—which included extended field training—men burned about 1,440 more calories per day than they ate, while women had a smaller daily deficit of around 750 calories. Term 2 also produced the lowest energy intake and the highest energy expenditure for both sexes, resulting in the most pronounced energy deficits across the training cycle.

In terms of body composition, lean mass remained statistically unchanged for both men and women throughout the 44-week course, indicating that participants were able to maintain muscle mass despite prolonged caloric deficits. Interestingly, fat mass and body fat percentage increased progressively from term 1 to terms 2 and 3 in both sexes.

Regarding metabolic and endocrine markers, several adaptive changes were observed. Leptin levels increased significantly in women over time but remained unchanged in men. In men, testosterone and free androgen index increased by term 3, suggesting a favorable hormonal response to training stress. Both sexes exhibited a decline in free T4 and a concurrent increase in thyroid-stimulating hormone (TSH), indicating subtle adjustments in thyroid regulation but not outright dysfunction. Cortisol levels decreased over time, and neither IGF-I nor total T3 showed significant changes, further supporting the conclusion that major endocrine disruption did not occur.

Interpretation & Significance

Despite undergoing extended periods of negative energy balance—particularly during intense field training—both men and women maintained stable lean mass and demonstrated adaptive hormonal responses. The maintenance of lean tissue in this study, despite substantial caloric restriction, suggests that cadets undergoing high-volume military training can preserve key physiological systems through a combination of structured physical activity, adequate protein intake, and intermittent recovery periods.

Moreover, the hormonal trends seen in both sexes appear more consistent with adaptive remodeling than with dysfunction. For example, increases in testosterone and TSH, as well as stable IGF-I and T3 levels, indicate that the body was effectively recalibrating rather than shutting down. 

Conclusions

This study demonstrates that while male cadets experience greater energy deficits than female cadets during prolonged military training, neither sex exhibited significant lean mass loss or metabolic/endocrine dysfunction. Instead, both groups showed physiological adaptations that supported stability under physical and nutritional stress. These results emphasize the importance of context-specific models when assessing energy availability and recovery in military populations. Rather than relying solely on frameworks derived from athletic performance, military nutrition and health strategies should account for the unique physiological resilience demonstrated in prolonged, high-stress training environments.

Source

1. O’Leary, T. J., O’Hara, J. P., Murphy, A. J., Pomeroy, V. M., Phillips, B. E., Stokes, K. A., Broom, D. R., & Mawer, H. E. (2024). Sex differences in energy balance, body composition, and metabolic and endocrine markers during prolonged arduous military training. Journal of Applied Physiology, 136(4), 938–948.